[PATCH v3] pseries/hotplug-memory: hot-add: skip redundant LMB lookup
Scott Cheloha
cheloha at linux.ibm.com
Thu Sep 17 00:39:13 AEST 2020
On Wed, Sep 16, 2020 at 09:39:53AM +0200, David Hildenbrand wrote:
> On 15.09.20 21:46, Scott Cheloha wrote:
> > During memory hot-add, dlpar_add_lmb() calls memory_add_physaddr_to_nid()
> > to determine which node id (nid) to use when later calling __add_memory().
> >
> > This is wasteful. On pseries, memory_add_physaddr_to_nid() finds an
> > appropriate nid for a given address by looking up the LMB containing the
> > address and then passing that LMB to of_drconf_to_nid_single() to get the
> > nid. In dlpar_add_lmb() we get this address from the LMB itself.
> >
> > In short, we have a pointer to an LMB and then we are searching for
> > that LMB *again* in order to find its nid.
> >
> > If we call of_drconf_to_nid_single() directly from dlpar_add_lmb() we
> > can skip the redundant lookup. The only error handling we need to
> > duplicate from memory_add_physaddr_to_nid() is the fallback to the
> > default nid when drconf_to_nid_single() returns -1 (NUMA_NO_NODE) or
> > an invalid nid.
> >
> > Skipping the extra lookup makes hot-add operations faster, especially
> > on machines with many LMBs.
> >
> > Consider an LPAR with 126976 LMBs. In one test, hot-adding 126000
> > LMBs on an upatched kernel took ~3.5 hours while a patched kernel
> > completed the same operation in ~2 hours:
> >
> > Unpatched (12450 seconds):
> > Sep 9 04:06:31 ltc-brazos1 drmgr[810169]: drmgr: -c mem -a -q 126000
> > Sep 9 04:06:31 ltc-brazos1 kernel: pseries-hotplug-mem: Attempting to hot-add 126000 LMB(s)
> > [...]
> > Sep 9 07:34:01 ltc-brazos1 kernel: pseries-hotplug-mem: Memory at 20000000 (drc index 80000002) was hot-added
> >
> > Patched (7065 seconds):
> > Sep 8 21:49:57 ltc-brazos1 drmgr[877703]: drmgr: -c mem -a -q 126000
> > Sep 8 21:49:57 ltc-brazos1 kernel: pseries-hotplug-mem: Attempting to hot-add 126000 LMB(s)
> > [...]
> > Sep 8 23:27:42 ltc-brazos1 kernel: pseries-hotplug-mem: Memory at 20000000 (drc index 80000002) was hot-added
> >
> > It should be noted that the speedup grows more substantial when
> > hot-adding LMBs at the end of the drconf range. This is because we
> > are skipping a linear LMB search.
> >
> > To see the distinction, consider smaller hot-add test on the same
> > LPAR. A perf-stat run with 10 iterations showed that hot-adding 4096
> > LMBs completed less than 1 second faster on a patched kernel:
> >
> > Unpatched:
> > Performance counter stats for 'drmgr -c mem -a -q 4096' (10 runs):
> >
> > 104,753.42 msec task-clock # 0.992 CPUs utilized ( +- 0.55% )
> > 4,708 context-switches # 0.045 K/sec ( +- 0.69% )
> > 2,444 cpu-migrations # 0.023 K/sec ( +- 1.25% )
> > 394 page-faults # 0.004 K/sec ( +- 0.22% )
> > 445,902,503,057 cycles # 4.257 GHz ( +- 0.55% ) (66.67%)
> > 8,558,376,740 stalled-cycles-frontend # 1.92% frontend cycles idle ( +- 0.88% ) (49.99%)
> > 300,346,181,651 stalled-cycles-backend # 67.36% backend cycles idle ( +- 0.76% ) (50.01%)
> > 258,091,488,691 instructions # 0.58 insn per cycle
> > # 1.16 stalled cycles per insn ( +- 0.22% ) (66.67%)
> > 70,568,169,256 branches # 673.660 M/sec ( +- 0.17% ) (50.01%)
> > 3,100,725,426 branch-misses # 4.39% of all branches ( +- 0.20% ) (49.99%)
> >
> > 105.583 +- 0.589 seconds time elapsed ( +- 0.56% )
> >
> > Patched:
> > Performance counter stats for 'drmgr -c mem -a -q 4096' (10 runs):
> >
> > 104,055.69 msec task-clock # 0.993 CPUs utilized ( +- 0.32% )
> > 4,606 context-switches # 0.044 K/sec ( +- 0.20% )
> > 2,463 cpu-migrations # 0.024 K/sec ( +- 0.93% )
> > 394 page-faults # 0.004 K/sec ( +- 0.25% )
> > 442,951,129,921 cycles # 4.257 GHz ( +- 0.32% ) (66.66%)
> > 8,710,413,329 stalled-cycles-frontend # 1.97% frontend cycles idle ( +- 0.47% ) (50.06%)
> > 299,656,905,836 stalled-cycles-backend # 67.65% backend cycles idle ( +- 0.39% ) (50.02%)
> > 252,731,168,193 instructions # 0.57 insn per cycle
> > # 1.19 stalled cycles per insn ( +- 0.20% ) (66.66%)
> > 68,902,851,121 branches # 662.173 M/sec ( +- 0.13% ) (49.94%)
> > 3,100,242,882 branch-misses # 4.50% of all branches ( +- 0.15% ) (49.98%)
> >
> > 104.829 +- 0.325 seconds time elapsed ( +- 0.31% )
> >
> > This is consistent. An add-by-count hot-add operation adds LMBs
> > greedily, so LMBs near the start of the drconf range are considered
> > first. On an otherwise idle LPAR with so many LMBs we would expect to
> > find the LMBs we need near the start of the drconf range, hence the
> > smaller speedup.
> >
> > Signed-off-by: Scott Cheloha <cheloha at linux.ibm.com>
>
>
> Hi Scott,
>
> IIRC, ppc DLPAR does a single add_memory() [...]
Yes.
> [...] for each LMB (16 MB).
The block size is set by the hypervisor. The default is 256MB. In
this test I had a block size of 256MB.
On multi-terabyte machines I would effectively always expect a block
size of 256MB. 16MB blocks are supported, but it is not the default
setting so it is increasingly rare.
> With tons of LMBs, this will also make /proc/iomem explode in size (using a
> list-based tree), making traversal significantly slower e.g., on
> insertions and system ram walks.
>
> I was wondering if you would get another performance boost under ppc
> when using MEMHP_MERGE_RESOURCE [1]. AFAIKs, the resource boundaries are
> not of interest. No guarantees, might be worth a try.
I'll give it a shot.
> Did you investigate what else makes memory hotplug that slow? (126000
> LMBs correspond to roughly 2TB, that shouldn't take 2 hours ...)
It was about ~31TB in 256MB blocks. It's a worst-case test (add all
the memory), but I'm pretty happy with a 1.5 hour improvement :)
> Memory block devices might still be a slowdown (although we have an
> xarray in place now that takes care of most pain).
Memory block devices are no longer a hotspot.
Some of the slowdown is in the printk overhead. We print a log for
every LMB. It is very silly. I intend to move those to a debug
priority, which should trivially speed things up.
Otherwise I need to do more profiling.
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